Electroosmosis was first described by Reuss in 1808 in his experiment on migration of waterthrough wet clay under the influence of an electric field. Electroosmosis in porous media, such as clays, is due to an electrical double layer of negative and positive ions formed at the solid-liquid interface. For clay particles, the double layer consists of a fixed layer of negative ions that are firmly held to the solid phase and a diffuse layer of positive ions that are more loosely held. Application of an electric potential on the double layer results in the displacement of the two layers to respective electrodes; i.e., the positively charged layer to the cathode and the negatively charged layer to the anode.
Since the clay particles are immobile, the fixed layer of the ions is unable to move. However, the diffuse layer containing positive or negative ions can move and drag water along with it to the respective electrode. This is the basic mechanism of electroosmotic transport of water through porous media under the influence of an applied electric potential.
Liquid flow rate and energy use in electroosmosis can be expressed by the following equations: ##EQU1## where Q =flow rate of water
D =dielectric constant of water PA1 .xi.=zeta potential PA1 I =current PA1 .mu.=viscosity of water PA1 .lambda.=electrical conductivity of cake PA1 E =energy use PA1 P =power input PA1 V =voltage. PA1 Nature of contaminant PA1 Concentration of heavy metals PA1 Soil type PA1 Ionic radius PA1 Solubility of contaminant as a function of pH PA1 Ease of release of contaminant from the soil PA1 pH control around the electrodes. PA1 Orthokinetic forces which cause small particles to agglomerate PA1 Bernoulli's force which causes larger particles to agglomerate PA1 Rectified Diffusion which causes gas bubbles to grow inside capillaries and thereby expel entrapped liquids PA1 "Rectified" Stokes, force which causes an apparent viscosity to vary nonlinearly and forces the particle towards the source PA1 Decreased Apparent Viscosity which may be due to high strain rates in a thixotropic medium or localized heating which in turn lowers both the viscosity and the driving force to move particles PA1 Radiation Pressure is a static pressure which is a second order effect adding to the normal pressure differential.
The flow rate is proportional to current and inversely proportional to conductivity. The energy use is proportional to voltage and electrical conductivity. High electrical conductivity results in high energy use and waste of energy by resistive heating.
The dominant mechanism of the enhanced flow is electroosmosis due to the electric field. In situ electroosmosis was first applied successfully to soils by L. Casagrande in the 1930's in Germany for dewatering and stabilizing soils. Casagrande, L., "Electroosmosis and Related Phenomena", Harvard Soil Mechanics Series No. 66 (1962); and Casagrande, L., "Review of Past and Current Work in Electroosmotic Stabilization of Soils", Harvard Soil Mechanics Series No. 145 (1957). Recently, Muralidhara and co-workers at Battelle have discovered that the simultaneous application of an electric field and an acoustic field produce a synergistic effect and results in further enhancement of water transport. Muralidhara, H.S., and D. Ensminger, "Acoustic Dewatering and Drying: State-of-the-Art Review," Proceedings IV, International Drying Technology Symposium, Kyoto, Japan, 1984. Muralidhara, H.S., and N. Senapati, "A Novel Method of Dewatering Fine Particle Slurries," presented at International Fine Particle Society Conference, Orlando, Fla., 1984. Muralidhara, H. S., et al., Battelle's Dewatering Process for Dewatering Lignite Slurries, Battelle Phase I Report to UND Energy Research Center/EPRI, 1985. Chauhan, S.P., H.S. Muralidhara, B. C. Kim, "Electroacoustic Dewatering of POTW Sludges", Proc. National Conf. on Municipal Treatment Plant Sludge Management, Orlando Fla., May 28-30, 1986. Muralidhara, H. S., et al., "A Novel Electro Acoustic Process for Separation of fine Particle Suspensions", Ch. 13, pp. 374, in Advances in Solid-Liquid Separation, Editor H.S. Muralidhara. Muralidhara, H.S., N. Senapati, and B.K. Parekh, Solid-Liquid Separation Process for Fine Particle Suspensions by an Electric and Ultrasonic Field, U.S. Pat. No. 4,561,953, December 1985. Senapati, N., H.S. Muralidhara and R.E. Beard on "Ultrasonic Interactions in Electro-acoustic Dewatering", presented at British Sugar Technical Conference, Norwitch, U.K., June 1988. Muralidhara, H.S., "Recent Developments in Solid-Liquid Separation", presented at the Trilateral Particuology Conference in Peking, China, September 1988. Beard, R.E., and H.S. Muralidhara, "Mechanistic Considerations of Acoustic Dewatering Techniques", Proc. IEEE, Acoustic Symposium, pp. 1072-1074, 1985. Muralidhara, H.S., Editor, Recent Advances in Solid-Liquid Separation, Battelle Press, Columbus, Oh. November 1986.
The electroosmotic flow is independent of the capillary diameter, a key advantage of electroosmosis over conventional flow under a pressure gradient. In the absence of an electric field, the flow of water through small pores essentially stops.
Some noteworthy examples of the prior work on soil leaching, consolidation, and dewatering by electroosmosis are presented below. Numerous patents have been issued in various applications of electric field for enhanced recovery of crude oil. Bell, T.G., U.S. Pat. No. 2,799,641 (1957). Faris, S.R., U.S. Pat. No. 3,417,823 (1968). Gill, W.G., U.S. Pat. No. 3,642.066 (1972). Bell, C. W., and Titus, C.H., U.S. Pat. No. 3,782,465 (1974). Kermabon, A.J., U.S. Pat. No. 4,466,484 (1984). Hardy, R.M., Unpublished presentation at NRC Canada, Ottawa, Canada (Dec. 1953). Banerjee, S., "Electrodecontamination of Chrome-Contaminated Soils", Land Disposal, Remedial Action, Incineration and Treatment of Hazardous Wastes, Proc. Thirteenth Annual Research Symposium, pp. 192-201 (July, 1987), Horng, J.J., BanerJee, S., and Hermann, J.G., "Evaluating Electrokinetics as a Remedial Action Technique", Second International Conference on New Frontiers for Hazardous Waste Treatment, Pittsburgh Pa. (Sept. 27-30, 1987). Anbah, S.A., et al., "Application of Electrokinetic Phenomena in Civil Engineering (S and Petroleum Engineering", Annuals, Volume 118, Art. 14, (1965).
According to Lageman, R., "Electro Reclamation in Theory and Practice", presented at Forum on Innovative Hazardous Waste Treatment Technologies' at Atlanta, Ga., June 19-21, 1989, the following factors play a key role in determining the efficiency of electrolysis process during heavy metal decontamination of the soil. These factors are:
An acoustic field is one in which pressure and particle velocity vary as a function of time and position. These fluctuations form a wave which propagates from the source throughout the medium. The pressure fluctuations may be sinusoidal and are characterized by their pressure amplitude and frequency. A particle velocity is imparted to the medium by the action of the pressure wave, and also varies as a function of time, frequency and position. Acoustic pressure and particle velocities are related through the acoustic impedance of the medium.
The pressure fluctuations are the result of the transmission of mechanical energy that can perform useful work to bring about desired effects. The type and magnitude of these effects depend on the medium. In acoustic leaching many of the forces which can contribute to the overall effectiveness. These include:
A more thorough review is available in the two articles by Ensminger and Muralidhara, et al. that appear in Advances in Solid-Liquid Separation above.
Many U.S. sites are contaminated with heavy metals such as zinc, mercury, cadmium, chromium arsenic, etc., and anion like cyanide. An object of the invention is to decontaminate soils containing heavy metals by the application of D.C. and acoustic fields.